Your system might be infected with viruses or malwares. Since you won't be able to run any executable files. I advise you to update your antivirus definition lists and to scan your system.

If you have already Ad-aware and Spybot S&D in your system, just update them and let them run at once.

The machine switches off at random. After being on for some time it can restart itself.it has had the tendancy of restating when loging on the internet. the antivirus on the machine is up to date. it updates automatically on the latest versions via the internet. could it be that the processor is overheating? I checked the processer fan and the power supply fan and both are working fine. Please give me another suggestion as to how I can resolve this.

I dont think the anti virus helps when is comes to spyware, adware stuff and your problem does look like one of those. Try couple of spyware programs like spyware doctor, vundufix etc. they will help you get rid of the problems.

But obviously if your PC keeps rebooting then you may have to back up all the data on your hard disk and do a clean install. But first try all other options ofscanning with spyware programs.

Regards
Raj

If your machines shutting down at random, it could be an overheating or overworked processor. It could be a dying powersupply. Could be a dying mobo. Check the event viewer logs and see if you have any weird entries.